ABSTRACT (Project 3: Maier, Chorover, Neilson, Molina-Freaner, Barton) Mine tailings are wastes leftover following the crushing of ores and extraction of metals such as copper during the mining process. This waste is devoid of the properties that make soils a suitable habitat for plants including soil structure, organic matter, nutrients and microorganisms. Of particular concern are legacy mine tailings which are often abandoned and are characteristically highly acidic with elevated levels of toxic metals such as arsenic and lead. Often these metals are associated with very small tailings particles that are easily windborne. These tailings deposits, which range from several to thousands of acres in size, can remain barren for decades or longer resulting in wind and water erosion into the surrounding environment especially in arid environments. Thus, these tailings have the potential to impact both human and environmental health in areas surrounding such waste sites. This project seeks to develop an alternative revegetation technology called ?compost-assisted phytostabilization? to allow lower-cost, long-term remediation of mine tailings, a major component of mine waste. Legacy mine tailings present toxic habitats for plant and microbial growth, preventing natural revegetation of these sites. The knowledge gap that impedes successful implementation of compost-assisted phytostabilization is a lack of understanding of how microbial and plant communities mediate the transition of mine tailings from plant-suppressing, acid-generating wastes to plant-sustaining substrates. The overall goal of this project is to develop a mechanistic understanding of the progression in microbial community development and plant-microbe interactions that allows this transition to take place. Our objectives are to: 1) define the type of microbes and associated functions that are responsible for acidification of legacy mine tailings; 2) identify key components of the nutrient cycling and plant growth promoting microbial communities that are present in stable plant ?fertility islands? on mine tailings at our Superfund field site, but absent from areas where plant establishment is failing; and 3) identify mechanisms of root-microbe-metal interactions in established plant fertility islands that immobilize contaminant metals during phytostabilization of mine tailings. This project will improve understanding of how components of the microbial community drive the success or failure of phytostabilization. Such information is essential to close the knowledge gap that prevents successful mine-tailing remediation of abandoned mine lands which jeopardize the health of neighboring communities and ecosystems. This research will also be concurrently translated to major mining companies to improve mine-tailing remediation practices.